Flame holders, also known as burners, are used in heating units to produce stable, evenly distributed flames without flashback or burnback. Flameholder design has a large impact on heating unit performance. Over the years, many different flame holder designs have been used or proposed. Especially in residential heating units, flame holder design has typically emphasized low manufacturing cost, durability and, for induced draft heating units, low pressure drop.
Durability is an important consideration because of the harsh environment inside a heating unit. Over its lifetime, a flame holder is exposed to thousands of thermal cycles, high temperatures, and the corrosive effects of combustion products such as nitric and carbonic acids. A durable flame holder allows a heating unit to operate reliably for long periods with low maintenance costs.
In induced draft heating units, fuel and combustion air are drawn through the flame holder by a fan located downstream of the combustion chamber. The size of the fan and related motor needed to move the fuel and air depend in large part on the pressure drop across the flame holder. To minimize fan and motor size, a low pressure drop flame holder is desirable. For example, a flame holder used in a residential, gas-fired, induced draft heating unit typically has a pressure drop less than 250 Pa (1 inch of water).
With the renewed focus on environmental issues, the impact of flame holder design on pollutant emissions is becoming an important consideration. Future flame holders will need to produce low levels of pollutants, such as CO and NO.sub.x, while still providing the desired durability and low pressure drop.
Therefore, what is needed in the industry is a heating unit with a durable, low pressure drop flame holder that produces low pollutant emissions.